Photoinitiator concentration is a fundamental parameter that determines the polymerization characteristics of a composite [7, 8]. This concentration is variable among the commercial composite and its effects on polymeric structure of composites is not still completely elucidated [23].
In this study the variation on concentration of photoinitiators affected all the evaluated properties, confirming the tested hypothesis. The composites with higher concentration of CQ/DMAEMA (C2 and C3) showed higher DC than the composite with lower concentration (C1). In addition, the depth of cure was also higher for C2 and C3 than for C1. What means that, the presence of a higher quantity of photoinitiators (CQ/DMAEMA) allows the generation of more radicals, which have resulted increased degree of conversion in some conditions of photocuring. Regarding the parameters analyzed, the most adequate photoinitiator concentration was 1.5 wt% of CQ/DMAEMA. The concentration of 0.5 wt% of CQ/DMAEMA was considered inappropriate for the resin matrix used.
The attempt to find an ideal concentration of photoinitiators for resin materials was the aim of a study from Moin Jan et al.[22]. Evaluating degree of conversion and photoinitiators elution, authors claimed that the better concentration of photoinitiators is 0.5% of CQ/DMAEMA in 1:1 proportion. These authors also found a decrease on the DC and increase in composite components elution (especially CQ) in experimental composites with 0.8 e 0.9% de CQ/DMAEMA. At first, this result seems to be divergent to ours, since in this study the composite with 0.5% of photoinitiators showed the worst results. However, the resin monomer system evaluated in Moin Jan et al.[22] study was UDMA/TEGDMA (50:50) and no filler particles were included. In the present study, the composite has 65% fillers and a monomer system based on BisGMA/TEGDMA (65:35). As UDMA has lower molecular weight and higher molecular mobility than BisGMA, and the quantity of TEGDMA (diluents monomer) is higher in the Moin Jan material, we could speculate that the Moin Jan material need less concentration of photoinitiators when compared to systems with higher molecular weight, as the system used in the present study. The filler particles also turn the polymerization more difficult, in this way the concentration of photoinitiators should be higher in composites than in resins.
The effect of photoinitiator concentration was also evident in the depth of cure evaluation. The depth of cure for C1 (1 mm, regardless the photoactivation method) was lower than that observed for C2 (2 mm) e C3 (2 mm / 3 mm). In this evaluation, was also verified a significant reduction on KHN with increase of depth for all materials. It is explained by the difficult of light penetration within the composite in the deeper layers, what leads to reduction on conversion degree [24]. Therefore, reduction on degree of conversion in these areas would reduce the KHN. This reduction on KHN was more evident for C1 (0.5% CQ/DAMEMA), which was completely unstiffened (not cured) at the depth of 4 mm. These results corroborate with Cunha et al.[25]. Also, according to Table 3, the hardness of the C3 in all depths was superior or similar to C2, but always higher than C1, what also reveal the superior effectiveness of in composites with higher concentration of photoinitiators.
The results of depth of cure are supported by the results of the DC test, in which the composite with 0.5% of CQ/DMAEMA (C1) presented significant less conversion than the others (C2 and C3). Also, the cross-linking density is increased for composites with higher photoinitiator concentration photoactivated using high irradiance. The combination C3 and HI provided the composite with the best characteristics of polymerization.
Thus, the first tested hypothesis, that the higher concentration of photoinitiator can increase degree of conversion, surface hardness, depth of cure and crosslink density was accepted. The exception occurred when the low concentration composite (C1) was photoactivated with LI, however, it should be considered that C1 showed lower DC when compared to other composites. This difference may explain the different behavior of softening in ethanol between C1 and C2/C3.
Another factor analysed in this study was the photoactivation method. Some studies have shown that photoactivation method can affect the polymerization characteristics of resin composites [10, 13, 20]. Modulated photoactivation methods can reduce shrinkage stress without interfere with properties such as volumetric shrinkage, degree of conversion, hardness, flexural strength, and others [13, 26, 27].
In this study photoactivation methods had no influence on degree of conversion, regardless the material, what corroborate whit other studies that employed different photoactivation methods with similar energy dose [13, 26, 27], validating part of the second tested hypothesis. Low intensity did not reduce degree of conversion because the light exposure time was increased until the polymerization reaches the diffusion limited free radical propagation [27]. It is because the polymerization is controlled by the radiant exposure. Some authors showed degree of conversion is directly related to the radiant exposure (Radiant exposure = Irradiance × Light exposure) delivered to the material [27, 28].
The depth of cure test revealed significant difference among the photoactivation methods for C3 denying the hypothesis that photoactivation methods do not affect the depth of cure. Regarding this material, methods HI and SS showed depth of cure of 3 mm while PD and LI, 2 mm. The likely explanation for this fact is that the long exposure time associated with the reduced irradiance in the first period of the activation increased the conversion at the superior layers and turned difficult the light diffusion within this 2 mm layer, reducing the conversion/KHN at 3 mm. Conversely, for the other photoactivation methods, the initial (for HI) or quick (for SS) exposure of the material to the higher irradiance, allowed easier light incoming into the 2 first mm and similar conversion/KHN on the 3 mm depth.
Differences in network structure determine the ethanol softening, what, after all, is determined by the cross-linking density [13, 20, 29]. The exposure contact of resin composites with the solvents causes the softening of the polymeric dimethacrylate materials because attraction forces between solvent molecules and polymer chain compounds are stronger than attraction forces among the polymeric chains. The solvent penetrates within the resin matrix and expand the openings among the chains, allowing elution of residual monomers and dissolution of the linear chain [29, 30]. The penetration ability of the solvent is related to its solubility parameter (ability of a molecule in penetrating and dissolving another substance). The difference in solubility parameters between polymers and solvents determines the softening of the polymer matrix: the lower the difference the higher the solvent penetration into the polymer network, the higher the softening [29, 30] For composites, solubility parameter is related to formulation (monomers type/concentration) and networking (cross-linking density). More linear polymeric chains would be more degraded by the ethanol than more crosslink chains [30]. It is because, solvent may form strong secondary bond with polymer chains, replacing secondary bonds among chains (dissolution of linear polymeric network). Such polymer-solvent secondary bond, however, cannot overcome primary valence cross-link, consequently the cross-linked polymers are less soluble, reducing the softening of the network. Additionally, cross-links prevent solvent molecules from establishing adequate interactions with the polymer thus preventing the molecules to be carried off into solution [30]. In this study, all groups showed, in more or less extension, significant degradation/swelling/softening after 24 h ethanol storage.
The photoactivation using high intensity seems to provide a polymeric matrix more resistant. The softening in ethanol of the specimen photoactivated using HI was significantly inferior to those observed in the specimen photoactivated using LI, for all materials. For C3, HI also showed significant reduction on softening when compared with SS and PD. As the radiant exposure has been the same (28 J/cm2) for all methods, this result suggests difference in cross-linking density among the methods. Possibly, the activation using continuous light in high intensity started multiple chain propagation centres, what generates higher degree of cross-linking [30]. Highly cross-linked polymers tend to be harder, inflexible, more heat-resistant and more fracture-resistant [31]. This result validates the second part of the second hypothesis.
Composites polymerized using low irradiance or using modulated photoactivation methods showed polymeric network less resistant to ethanol softening, which corroborate with previous studies, in which activation using low intensity (just initially or during the total light exposure) were associated to the fewer chain propagation centres, allowing the formation a more linear network [13, 20, 30]. Probably, this network presents less cross linking and more primary ciclyzation. The primary cyclization reactions create micro-gels and lead to heterogeneity in the polymer network where loosely cross-linked regions and more highly cross-linked micro-gel regions coexist. This cyclization will promote higher local conversion as it does not decrease the mobility of the system as much as cross-linking. However, cyclization can also lead to a reduction in the effective cross-linking density, since cycles do not substantially contribute to the overall network structure. According to Brandt et al.[13], irradiance, more than radiance exposure, determines the behaviour of the composite after ethanol storage.
Another aspect to discuss is depth of cure reached by specimen photoactivated using low irradiance. Cunha et al.[25] stated that the hardness not only depends on the amount of energy but also on the irradiance. Considering that, if the irradiance is reduced at the surface, it could be expected a more intense reduction on this intensity when the light passes through the composite. So, it could be expected that the method that uses low intensity (LI) would present a more accentuated reduction on KHN at the deeper areas than the other methods. However, it not occurred. As the exposure radiant was adjusted the all photoactivation methods, the polymerization depth obtained for all methods was quite similar. Therefore, it could be hypothesized that the regulating factor for depth of cure is the radiant exposure, and if this dose were adjusted even using a low intensity, as 150 mW/cm2, the polymerization will have a pattern similar or even better than that observed for other PM.
According to the results of this study, composites with different concentration of photoinitiators show different polymerization behaviour. And, despite of the benefits of stress reduction obtained with modulated photoactivation methods or low-intensity method, it should be considered that there is a possibility of decrease on mechanical properties and low resistance to chemical degradation of the polymer matrix due to the formation of a more linear structure (low cross-linking). Thus, only the understanding of the causes of problems associated to development of techniques to reduce their consequences would help the clinicians to obtain the maximum benefits of composite restorations in clinical practice.